A copolymer of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) has been commercially available for many years. The copolymer (sometimes referred to as TFE/HFP hereinafter) is noted for its stability at high temperature, chemical resistance, and excellent electrical properties. Similarly, a copolymer of TFE and perfluoro(propyl vinyl ether) (PPVE), (TFE/PPVE hereinafter), possesses all the favorable properties of TFE/HFP and in addition can be used for extended periods of time at even higher temperatures than TFE/HFP. A terpolymer of TFE, HFP and PPVE, has high temperature properties, such as tensile strength, intermediate between TFE/HFP and TPE/PPVE).
A major use of TFE/HFP is as insulation and jacketing of wire and cable which is manufactured by applying the molten copolymer onto a rapidly moving wire by means of an extruder running at high temperature using a specially designed die. In order to maximize the line speed of the wire coating line it is advantageous to have the viscosity of the molten polymer be as low as possible in order to maximize the extrusion rate. However, as the melt viscosity (MV) of the TFE/HFP or TFE/PPVE copolymer is lowered, the stress crack resistance (SCR), conveniently measured by the MIT flex life, is also lowered. Low stress crack resistance is manifested by cracks occurring in the wire or cable insulation after cooling, perhaps as long as several years later. SCR is similarly important for other important applications of TFE/HFP such as linings for tanks and valves, tubing, film etc.
It would be of great benefit to increase the stress crack resistance of TFE/HFP at the same MV, or alternatively to lower the MV of TFE/HFP to allow a faster extrusion rate while keeping the SCR at a high level.